Carrier current signaling system



Nov. 11, 1947. A. G. LANG CARRIER CURRENT SIGNALING SYSTEM Filed Mayvl, 1946 2 Sheeos-Sl'leet` l A. 6. LANG ATTORNEY Nov. l1, 1947. A. G. LANG CARRIER CURRENT SIGNALING SYSTEM R 2 Sheets-Sheet 2 Filed May 5l, 1946 HHIILMUH... R

kmh Sk /NVENTOR A. G. ANG 6X9. @M

ATTORNEY Patented Nov. 11, 1947 CARRIER CURRENT SIGNALING SYSTEM Albert G. Lang, Whitestone, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 31, 1946, Serial No. 673,270

6 Claims.

This invention relates to communication systems and particularly to telephone systems including subscribers stations connected to the telephone exchange over a power line.

An object of the invention is the prevention of bell tapping at telephone subscriber stations which are arranged for carrier current transmission of speech and signaling circuits.

This invention is an improved signaling circuit arrangement at a telephone subscriber's station which is connected for carrier current operation over a power line. A feature of the invention is the provision of means for preventing bell tapping responsive to static disturbances on the power line or to carrier current received over the power line.

A clear and complete description of the invention will be facilitated by considering a system in which the invention is embodied, one such system being illustrated schematically in the drawing. The invention is not limited to the particular system shown, but is generally applicable to telephone systems arranged for carrier current signaling.

The drawing, which consists of two figures, shows schematically an automatic telephone system comprising telephone subscribers stations connected to the telephone exchange by a power line and arranged for carrier current operation over the power line.

Fig. 1 shows a telephone subscribers station A connected by a line LI to a central oflice CO comprising a line finder LF, selector S and connector C. Fig. 1 further shows another subscribers line L2 having associated therewith carrier terminal equipment CT which is connected through a coupler 68 to a power line PL; and

Fig. 2 shows the power line PL connected to each of a plurality of telephone subscriber stations B, C, D, the station B being shown in detail.

The subscribers station A is of the usual type provided in common battery telephone systems and includes a dial for use in establishing desired connections. The line circuits LCI and LCZ associated with lines LI and L2, respectively, each consists of a line relay and a line cut-off relay as in the line circuit disclosed in the patent to R. L. Stokley, 1,799,654, April 7, 1931. The line finder, selector and connector switches are of the two-motion step-by-step type; and reference may be had to Automatic Telephony by Smith and Campbell, second edition, published in 1921 for a description of the structure of such switches and their operation as selectors and connectors. Reference may be had to the aforementioned patent to R. L. Stokley for a description of the operation of a step-by-step line finder. Each of the switches LF, S and C is represented by a set of brushes and a single set of bank terminals, the relays and other apparatus associated with the line nder LF and selector S being entirely omitted and the relays and other apparatus associated with the connector C being shown only to the extent that is necessary for a clear and complete explanation of the invention. Omitted apparatus is represented by broken lines. The connector C is arranged for ten-party code ringing, and includes a coderinging selector comprising brushes 3l and 32 for selecting any one of ve ringing code supply conductors from the ringing machine. Reference may be had to the patent to C. D. Koechling 2,289,503, granted July 14, 1924, for a complete description of the operation of such a connector.

The carrier terminal apparatus CT, associated with the line L2, is similar to the like designated apparatus in the copending patent application Serial No. 653,254, led March 9, 1946, by R. C. Edson and J. W. Emling. The apparatus CT comprises a modulator 45 for transmitting voice and signal modulated carrier current of a frequency Fl over the power line on all calls; a demodulator 52 for voice modulated and signal modulated carrier current of frequency F2 and a demodulator 56 for voice modulated carrier current of frequency F3. Reference may be had to the Edson-Emling application for a complete description of the apparatus CT, the description herein being limited to that required for a clear and complete understanding of the invention. The conductors of line L2 are connected to one pair of windings of hybrid coil 39, through which voice and signalirg currents are transmitted from line L2 to the modulator 45 and from demodulator 52 to line L2.

The subscriber station B and each of` the other stations is similar to station A disclosed in the aforementioned Edson-Emling application, but differs therefrom in some respects including the ringing arrangement. The power line PL is connected through coupler 69 to the plug-in pad lil. The subscrbers station comprises a ringer 85, a handset 88, and associated switch 89, and an impulse dial of usual type for use in establishing desired connections. The station further comprises an induction coil 9| and modulator, amplier and band filter apparatus represented by the enclosure 94 for transmitting voice and signaling current over the power line PL from station B. The station further comprises band lter and amplifier apparatus represented by the enclosure '|2 responsive to voice and signaling current incoming over the power line PL, a demodulator '|3, signaling relays R| and R2, and a busy indicating lamp 84. The station is provided with a suitable source of energy which is represented as being an alteranting current source 95, transformer 96 and rectifier 91. The source 95 may be the power line PL or an intermediate transformer connected thereto.

Assume now that a. call is initiated at station A, that the line finder LF operates to extend the line L| to the selector S, that the selector S is operated under the control of the dial at station A to select a group of connectors and is further operated to select an idle connector C in the selected group. When the calling subscriber dials the last three digits of the called subscribers directory number, the brushes of connector C are advanced to engage the terminals of the called line and the code selector brushes 3| and 32 are advanced to the position corresponding to the ringing code to be used in signaling the called subscribers station. Assume further that the called station is the station B shown in Fig. 2, that the brushes of connector C are advanced to engage the terminals to which line L2 is connected and that brushes 3| and 32 are advanced to engage terminal l in which position code ringing current is transmitted over the upper conductor of line L2. The connection through switches LF and S is maintained under the control of line relay and slow release relay l2 of connector C in usual and well-known manner. The pick-up relay 2| is operated over the pickup conductor PU at the beginning of a ringing cycle, thereby closing the ringing circuit from the code I ringing conductor through brush 3|, a front contact of relay 2|, outer back contact of ringing relay I6, brush 55 and engaged terminal, one winding of hybrid coil 39, a back contact of relay R6, full wave rectifier 51 and winding of relay R5, to ground. Relay R5 is thereby operatively energized. Relay R5 connects battery to energize modulator 45 and ampliiier lll', and extends the ringing circuit to modulator 45; whereby carrier current, modulated by the ringing current, is transmitted through amplifier l1 and transmitting band lter 48, through coupler 68, over power line PL to all of the stations connected to the power line.

'Ihe ringing modulated carrier current incoming over the power line PL to each of the stations is transmitted through a coupler 69, plug-in pad 10, input conductors receiving band lter and amplier apparatus 12, to a demodulator '|3. At each of the stations, the winding of relay Rl is connected, through a resistor '|5 and condenser 'l5 in parallel, to the demodulator output; whereby relay Rl at each station is operatively energized and deenergized at a 20-cycle rate by the demodulated ringing current. Each operation of relay RI closes a circuit for operatively energizing relay R2, this circuit being traced from ground through the front contact of relay RI, resistor 82, winding of relay R2, normally closed contact of handset switch 89, to the energy supply conductor 98. erating to reduce the probability of both of relays RI and R2 being operated responsive to static disturbances on the power line. The operation of relay R2 connects its winding to conductor 98 independent of switch 89 so as to hold relay R2 operated after the handset is removed Relay R2 is somewhat slow in op-I from the hook, closes a circuit for lighting busy lamp 84, and opens the shunt through resistor normally connected across ringer 85 thereby to render the ringer operative. The condenser 51, which is initially charged from supply conductor 98 through resistor 86 in parallel with ringer 85, is discharged through resistor 8| as soon as relay Rl releases. Thereafter the alternate operation and release of relay Rl, at a 20-cycle rate responsive to ringing current, effects the alternate charge and discharge of condenser 81 through ringer 85, thereby operating the ringer according to the ringing code of the called station, the ringer at each of the stations being operated in like manner. Relay R2 is slow to release and remains operated while the ringer is being actuated during the ringing intervals of each ringing cycle. During the silent intervals of each ringing cycle, relay R5 releases thereby deenergizing the modulator l5 of carrier terminal apparatus CT and deenergizing relays RI and R2 at each of stations connected to the power line PL. When the call is answered at the called station by removing the handset from the hook, supply conductor 98 is connected through the lower front contact of the switch 89 to the modulator and amplier apparatus 94 whereby carrier current of either frequency F2 or F3 is supplied to modulator 9| depending upon whether relay R2 is, or is not, operated. In any event, as soon as there is a silent interval in the ringing cycle, relays RI and R2 release and with the handset removed, carrier current of frequency F2'is transmitted from modulator Sil through pad T9, coupler S9, over power line PL, through coupler 88, lter 69 and amplier 59 to demodulator 52, thereby effecting the operative energization of relay R3. The operation of relay R3 closes a short circuit across the windings of coil 39 which are connected to line L2 so as to operatively energize the lower winding of relay I8 of connector C, thereby to open the ringing circuit and complete the talking path through brushes 55 and 55 between the calling subscribers line Ll and the called line L2. The aforementioned operation of relay RS also closes a circuit for operating relay R6. Relay R8 opens the connection between ringing relay R5 and line L2 and connects battery'to modulator :l5 and amplifier 41, whereby carrier current of frequency FI is transmitted over the power line to again energize relay RI at each of the stations. At the called station B, the operation of relay RI does not eiect the op eration of relay R2 because the handset has been removed from the hook. At each of the other stations the R2 relay operates, but the ringer is not operated since the incoming carrier current is at this time modulated by voice currents from station A instead of by ringing current so that relay R| remains continuously operated. At the called station B, the transmitter of handset 83 is connected by switch 89 through coil 9| to modulator 94, the voice currents being effective to modulate the carrier current of frequency F2 transmitted over the power line, through coupler 68, lter 49, amplifier 58, to demodulator 52. When the conversation is nished, the return of the handset at the called station B to the hook of switch 89 deenergizes the modulator 92, and the return of the instrument to normal at the calling station causes the successive release of line relay and release relay I2 of connector C. The line nder LF, selector S and connector C are thereupon returned to normal in usual and well-known manner. The carrier terminal relay R3 releases when the modulator 94 is deenergized at station B; and the release of relay R3 `causes the release of relay Rt, thereby deenergizing the modulator d and amplier lll.

On a call initiated at station B, carrier current of F2 is transmitted from modulator 9| responsive to removal of the handset 83, thereby causing the operation of relay R3 and energization of modulator 45, as above described. The carrier current of frequency Fl thereupon transmitted from modulator 45 causes the operation of relays Rl and R2 at each of the other stations C, D, so as to light the busy lamp 84 at each of these stations. The operation of relay R3 also closes the short-circuit across the windings of coil 39 which are connected to line L2 to operate the line relay of line circuit LC2 and thus effect the extension of the call through a line nder LF to a selector S. The call is further extended through the selector and a connector to a called line under the control of the dial at the calling station B. If the called station is one of the other stations connected to the power line PL, the handset at station B is replaced on the hook to start ringing. When the called station answers, relays R3 and R6 of carrier terminal equipment CT are operated as above described on a call initiated at station A, and carrier current of frequency FI is thereupon transmitted from modulator 45 to energize relays RI and R2 at the calling station B and also at each of the other stations except the called station. When the handset is removed at the calling station B, the modulator et thereat is energized to transmit carrier current of frequency F3 over the power line to operate relay R4. Relay R4 locks independent of relay R3, and connects the output of dernodulator 56 in parallel with the output of demodulator 52 to the lower winding of coil 39, whereby two-way conversation is carried on through the demodulators 52 and 5S and modulator l5 over power line PL between the calling station B and the called one of the other stations C, D. When the handsets are restored to normal at the calling and called stations, the modulators SQ thereat are deenergized, relays R3, Ril and R6 of the carrier terminal equipment CT are restored to normal, the shortcircuit across line L2 is opened by the release of both of relays R3 and R4, and the connector C is thereupon restored to normal in usual and wellknown manner,

In order to prevent bell tapping at any power line carrier station, when relay Rl is energized by carrier current which is not modulated by ringing current, the ringer 85 at each station is normally shunted by a resistor 86 and the ringer is poled so that it is less sensitive to current discharging condenser 'l than to current charging condenser 8l. The current which charges condenser S's', when relay Rl operates and relay R2 is normal, is thus divided between resistor 86 and ringer 85, so that the ringer will not be suffi-- ciently energized to tap the bell responsive to the initial energization of relay Rl. When relay R2 operates, the path through resistor 8G is opened so that the alternate release and reoperation of relay RI responsive to ringing current causes the alternate discharge and charge of condenser 8l through ringer 85 alone so that the ringer is actuated according to the ringing code. This shunt, normally connected across the ringer 85 at each station, is also effective to prevent bell tapping responsive to operation of relay Ri by power line disturbances which may occur with sufhcient frequency to be annoying if allowed to cause opera- 6 tion of the ringers.; and if relay Ri is held operated bya static disturbance long enough to effect the operation of relay R2, the release of relay RI will be ineffective to tap the bell due to g the poling of the ringer.

What is claimed is :V

1. In a signaling system, a iirst relay, a ringer, a condenser, means for operating and releasing said first relay responsive to carrier current modulated by ringing current, means controlled by said first relay for charging and discharging said condenser through the winding of said ringer, a shunt normally closed across said ringer to prevent the operative energization of said ringer, and means comprising a second relay controlled by said `iirst relay for opening said shunt.

2. In a signaling system, a first relay, a ringer, a condenser, means for operating and releasing said first relay at a loW rate, means comprising said relay for alternately charging and discharging said condenser in series With the winding of said ringer, a shunt normally closed across said winding to prevent the operation of said ringer responsive to the initial operation of said rst relay, and relay means responsive to the initial operation of said first relay for opening said shunt and preventing the reclosure of said shunt while said first relay is being operated and released at said low rate.

i 3. In a telephone system, a subscribers station, means for transmitting carrier current to said station, said carrier current being at times modulated by ringing current of a low frequency, a relay at said station actuated continuously responsive to carrier current alone and intermittently responsive to carrier current modulated by ringing current, a ringer and a condenser at said station, means comprising the contacts of said relay for alternately charging and discharging said condenser through the winding of said ringer responsive to carrier current modulated by ringing current, and means comprising a shunt normally connected across said ringer for preventing the operative energization of the ringer responsive to a single energization of said relay.

4. In a signaling system, a relay, a ringer, a condenser, means responsive to signaling current for operating and releasing said relay at a low rate, means comprising the contacts of said relay for alternately charging and discharging said condenser in series with the winding of said ringer, and means comprising a resistor normally connected in parallel with the winding of said ringer for preventing the operative energization of the ringer responsive to a single energization of said relay.

5. In a signaling system, a relay, a ringer, a condenser, means responsive to signaling current for operating and releasing said relay at a low rate, means comprising the contacts of said relay for alternately charging and discharging said condenser in series with the winding of said ringer, means comprising a resistor normally connected in parallel with the winding of said ringer for preventing the operative energization of the ringer responsive to a single energization of said relay, and relay means controlled by said relay for opening the circuit path through said resistor to render said ringer operatively responsive to the alternate release and reoperation of said relay at said low rate.

6. In a signaling system, a relay, a polarized ringer, a condenser, means responsive to signaling current for alternately operating and releasing said relay, means comprising contacts of said relay for charging and discharging said condenser in series With the Winding of said ringer, means normally shunting said ringer to prevent the operative energization of said ringer, and means controlled by said relay for opening said shunt a predetermined interval of time after 8 said relay operates and for maintaining said shunt open While said relay is being operated and released responsive to signaling current, the poling of said ringer being such that the release of said relay is ineffective to operatively energize said ringer.

ALBERT G. LANG. 

